1. Field of the Invention
This invention relates to an additive for a linear saturated polyester molding composition; more particularly, this invention relates to a nucleating agent which is the reaction product of a low viscosity linear saturated polyester and a compound having at least one functional group selected from the class consisting of anhydrides, epoxides, and hydroxyls.
2. Description of the Prior Art
It is known in the art to use nucleating agents in crystallizable polymers such as linear saturated polyesters of aromatic dicarboxylic acids. A variety of nucleating agents have been disclosed. Included are inert particulate nucleating agents as well as a variety of salts of hydrocarbon and polymeric carboxylic acids.
U.S. Pat. No. 3,843,615 discloses a linear saturated polyester composition containing from 0.05 to 2% by weight of a cross-linked saturated polyester of terephthalic acid or an ester forming derivative thereof, a diol having from 2 to 10 carbon atoms, and 0.01 to 3% by weight calculated on the cross-linked saturated polyester of at least one cross-linking polyfunctional compound. The cross-linking polyfunctional compound has more than two groups capable of forming esters. Such groups include acids, anhydrides and epoxides. The cross-linked saturated polyester is crosslinked so that it is insoluble in a mixture of phenol and tetrachloroethane having a ratio of 3:2.
A molding formulation based on a linear saturated polyester, such as polyethylene terephthalate, should result in molded product having good physical properties including flexural strength, modulus, tensile strength and impact properties. Further, the molding compound should also have good molding properties, including a melt flow index for sufficient flow into the mold, good mold release properties and good finished appearance.
From a practical point of view, it is desirable that satisfactory properties, as outlined above, be attained using water heated molds. That is, molds heated to temperatures between 76.7.degree. C. (170.degree. F.) to about 113.degree. C. (235.degree. F.). In order to accomplish this, it is desirable for crystallization to begin at as high a temperature as possible upon the cooling of the polyester which was fed into the mold, and continue during the cooling to as low a temperature as possible. T.sub.cc is a measurement to determine at what temperature crystals first appear upon cooling from the melt. T.sub.ch is a measurement which indicates the temperature at which crystallization is no longer occurring upon cooling. It has been found that mold appearance and mold release properties can be related to T.sub.ch. T.sub.ch is determined by measuring the temperature at which crystals appear upon heating an amorphous piece of polyester. T.sub.cc and T.sub.ch can be measured using a Differential Scanning Calorimeter.